1. Field of the Invention
This invention relates to a vibration-damping resin composition which exhibits excellent damping properties at ambient temperature and low temperatures while having high mechanical strength, and a molded article made of such vibration-damping resin composition.
2. Description of the Related Art
In recent years, vehicle parts have been in practice changed from metallic parts to plastic parts to meet the social demand for weight reduction leading to fuel saving. Of various plastic materials, polypropylene resins have been used in large quantities in interior and exterior vehicle trim parts because of their well-balanced physical properties and relative inexpensiveness.
In particular, reinforced polypropylene resins comprising polypropylene resins loaded with inorganic fillers, such as talc, glass fiber, calcium carbonate and mixtures thereof, have high strength and heat resistance and are used in bumpers, body side moldings, dashboards, timing belt covers, blower cases, cooler housings, ducts, and the like.
In addition to weight reduction, reduction of noise of vehicles has also been demanded to improve comfort. Countermeasures against noise include vibration damping, sound insulation, vibration insulations, and sound absorption. For the noise of air conditioning equipment, different countermeasures are taken depending on whether the noise is chiefly transmitted through solid or air. Where noise is to be abated by material displacement, it is necessary to use materials having improved damping properties.
Conventional polypropylene resin compositions with improved damping properties include the one disclosed in Japanese Patent Publication No. Sho. 62-43443. The composition disclosed is still insufficient in satisfying both damping properties and mechanical strength when used in a temperature range from low temperatures to ambient temperature, e.g., from xe2x88x9240xc2x0 C. to +40xc2x0 C.
By the way, among the aforementioned exterior vehicle trim parts made of resins, there is a rocker-panel molding, which is attached to the lower side body either directly or via a rocker panel, is generally used for improvement of automobile appearance, protection of a car body from scratches or improvement of aerodynamic characteristics.
The conventional vehicle rocker-panel molding comprises a bottom section and a side section almost perpendicular to the bottom section, showing a nearly L-shaped crosswise profile. The rocker-panel molding should have rigidity to some extent because drivers or passengers may touch them on getting in or out. For the sake of convenience in molding, i.e., in order to obtain a uniform flow of a resin molding material through a mold, conventional rocker-panel moldings have a uniform thickness in the bottom section and the side section. That is, in conventional rocker-panel moldings the side section has a prescribed thickness, e.g., 3 mm, so as to secure necessary rigidity, and the bottom section has the same thickness as the side section.
When a spinning tire sends gravel or sand flying and striking against a rocker-panel molding especially the bottom section thereof, the impact generates vibration energy. Conventional rocker-panel moldings having the above-mentioned structure often have insufficient performance in reducing the vibration energy. The vibration energy produces noise (impact sound), which is transmitted from the rocker-panel molding through a rocker panel to the body, resulting in noise in the space of the car through pneumatic transmission.
An object of the present invention is to provide a damping resin composition which exhibits excellent damping properties and high mechanical strength in a low to ambient temperature range.
Another object of the present invention is to provide a rocker-panel molding which can suppress transmission of noise generated by collision of foreign matter against it into the space of the car without impairing its own function.
The present invention provides a damping resin composition comprising 100 parts by weight of a polymer blend of (A) 40 to 70% by weight of a polypropylene resin and (B) 30 to 60% by weight of a thermoplastic elastomer and 5 to 60 parts by weight of an inorganic filler, the thermoplastic elastomer (B) comprising (B1) at least one member selected from the group consisting of an elastomer having a glass transition temperature (hereinafter abbreviated as Tg) of 0xc2x0 C. to 20xc2x0 C. (hereinafter referred to as elastomer B11) and an elastomer having a Tg of xe2x88x9220xc2x0 C. to 0xc2x0 C. (hereinafter referred to as elastomer B12) and (B2) an ethylene-xcex1-olefin copolymer.
What is to be noted in the present invention is as follows: A damping resin composition contains: a polymer blend comprising (A) a polypropylene resin and (B) a specific thermoplastic elastomer; and an inorganic filler at a specific mixing ratio and that the thermoplastic elastomer (B) comprises (B1) an elastomer having a specific Tg and (B2) an ethylene-xcex1-olefin copolymer.
The polypropylene resin (A) which can be used in the present invention is preferably a crystalline polypropylene resin, including a general polypropylene resin and a modified polypropylene resin.
The general polypropylene resin includes a propylene homopolymer, a propylene-ethylene random copolymer (ethylene content: 20% by weight or less), a propylene-ethylene block copolymer (ethylene content: 20% by weight or less), and a mixture thereof. The modified polypropylene resin is a resin obtained by modifying a polypropylene resin with an unsaturated carboxylic acid or a derivative thereof in the presence of an organic peroxide.
It is preferable to use, as the crystalline polypropylene resin, a mixture of the general polypropylene resin and the modified polypropylene resin. In this case, the modified polypropylene resin is preferably used in a proportion of 1 to 20% by weight based on the total polymer blend. It is preferred for the crystalline polypropylene resin to have a melt flow index of 5 to 100. If the melt flow index is less than 5, the resin composition tends to have poor molding properties to provide molded articles with poor appearance. If it exceeds 100, the molded articles tend to have reduced impact strength.
The resin matrix (polymer blend) of the composition according to the present invention comprises 40 to 70% by weight of the polypropylene resin (A) and 30 to 60% by weight of the thermoplastic elastomer (B) hereinafter described in detail. If the proportion of the polypropylene resin is less than 40% by weight, mechanical strength is lessened. If it exceeds 70% by weight, the damping properties are reduced. If the proportion of the thermoplastic elastomer is less than 30% by weight, the damping properties are reduced. If it exceeds 60% by weight, mechanical strength is reduced.
The inorganic filler is added in an amount of 5 to 60 parts by weight per 100 parts by weight of the polymer blend. If its proportion is less than 5 parts by weight, the resin composition has insufficient mechanical strength. If it is more than 60 parts by weight, the damping properties and impact resistance are reduced.
The inorganic filler which can be used in the present invention includes talc, mica, clay, silica, alumina, calcium carbonate, magnesium carbonate, zinc oxide, titanium oxide, calcium oxide, magnesium oxide, calcium hydroxide, magnesium hydroxide, aluminum hydroxide, calcium sulfate, barium sulfate, basic magnesium sulfate, calcium silicate, glass, potassium titanate, and aluminum borate. These inorganic fillers can be in the form of fiber, whisker, flake or powder.
The thermoplastic elastomer (B) comprises an elastomer (B1) having a specific Tg and an ethylene-xcex1-olefin copolymer (B2)
The ethylene-xcex1-olefin copolymer (B2) includes ethylene-propylene rubber (EPM), ethylene-butadiene rubber (EBM), and ethylene-octene rubber (EOM).
The elastomer (B1) having a specific Tg is an elastomer B11 having a Tg of 0xc2x0 C. to 20xc2x0 C., an elastomer B12 having a Tg of xe2x88x9220xc2x0 C. to 0xc2x0 C. or a mixture of the elastomer B11 and the elastomer B12. The elastomer B11 (Tg: 0xc2x0 C. to 20xc2x0 C.) is particularly effective in improving the damping properties at xe2x88x9220xc2x0 C. to 40xc2x0 C. If the Tg is lower than 0xc2x0 C., damping properties may tend to be insufficient in ambient temperature around 40xc2x0 C. If the Tg is higher than 20xc2x0 C., the damping properties of the resin composition tends to be reduced around xe2x88x9220xc2x0 C. The elastomer B12 (Tg: xe2x88x9220xc2x0 C. to 0xc2x0 C.) is particularly effective in improving the damping properties at xe2x88x9240xc2x0 C. to 20xc2x0 C. If the Tg is lower than xe2x88x9220xc2x0 C., the resin composition tends to have reduced damping properties in ambient temperature around 20xc2x0 C. If the Tg exceeds 0xc2x0 C., the damping properties may tend to be reduced in temperatures below xe2x88x9240xc2x0 C.
The damping resin composition according to the present invention shows excellent damping performance in a low to ambient temperature range as having a damping tanxcex4 of 0.05 or more in a temperature range of from xe2x88x9240xc2x0 C. to 40xc2x0 C. and excellent mechanical strength as having a flexural modulus of 700 MPa or more, a heat distortion temperature (HDT) of 70xc2x0 C. or higher, and an Izod impact strength (at xe2x88x9230xc2x0 C.) of 30 J/m or more as demonstrated in Examples hereinafter given.
In a preferred embodiment of the present invention, a styrene-isoprene-styrene copolymer is used as the elastomer B11 (Tg: 0xc2x0 C. to 20xc2x0 C.). The vinyl structure of the polyisoprene block in the styrene-isoprene-styrene copolymer can have a 1,2-vinyl bond as shown in formula (I) or a 3,4-vinyl bond as shown in formula (II).
This elastomer has rubbery elasticity as well as thermoplasticity and is compatible with various plastics. 
In another preferred embodiment of the present invention, it is preferred to use a styrene-isoprene-styrene copolymer or polybutene-1 as the elastomer B12 (Tg: xe2x88x9220xc2x0 C. to 0xc2x0 C.) These elastomers have good compatibility with polypropylene resins.
In still another preferred embodiment, the polypropylene resin contains a modified polypropylene, modified with an unsaturated carboxylic acid or a derivative thereof. In this case, the modified polypropylene resin mediates between the polypropylene resin and the inorganic filler to improve mechanical properties such as elastic modulus, heat resistance and impact resistance. Examples of suitable unsaturated carboxylic acid or derivatives thereof are fumaric acid, citraconic acid, glutaconic acid, maleic acid, itaconic acid, maleic anhydride, glutaconic anhydride, and citraconic anhydride.
In yet another preferred embodiment of the present invention the polymer blend further comprises an oil extended ethylene-propylene-diene copolymer (EPDM). In this case, the resin composition has not only further improved damping properties but improved parting properties in molding to provide molded articles with improved appearance. The resin composition according to this embodiment is particularly effective when molded into long and narrow articles, such as bumpers, side body moldings, and rocker panel moldings.
The damping resin composition can be produced by compounding the above-described components by means of usual mixing machines, such as a single screw extruder, a twin screw extruder, a kneader, a Banbury mixer, etc. Usually, the components are mixed at a prescribed mixing ratio in a tumbling mixer, a Henschel mixer, a ribbon mixer, etc., and the resulting mixture is kneaded in an extruder, etc. and pelletized. The pellets can be molded into desired articles.
If desired, the damping resin composition can contain various additives, such as antioxidants, ultraviolet absorbers, lubricants, antistatic agents, nucleating agents, pigments, flame retarders, extenders, and processing aids.
The damping resin composition of the invention exhibits excellent damping properties particularly in a temperature range of from low temperatures to ambient temperature, e.g., from xe2x88x9240xc2x0 C. to 40xc2x0 C., and high mechanical strength. Therefore, it is useful in automotive parts and interior and exterior automotive trim parts and is effective In reducing the noise attributed to transmission of vibrations through a car body in a broad temperature range. For example, molded articles obtained from the damping resin composition of the present invention are effective in reducing transmission of noise produced when, for example, sand or gravel is sent flying and striking against the molded articles thereby to abate the noise in the space of a car.
The damping resin composition of the invention is also suitable as various fittings in factories or buildings or other electrical parts and machine parts.
Further, the above another object is accomplished by a vehicle rocker-panel molding made of the aforementioned damping resin composition, which is attached to the lower part of a vehicle side body and has a bottom section and a side section, the side section being almost perpendicular to the bottom section and possessing prescribed rigidity, wherein the bottom section has a function as an energy conversion means for forcibly converting vibration energy into heat energy.
In preferred embodiments of the present invention, the function as an energy conversion means is achieved by (1) making the bottom section thinner than the side section, (2) bonding a member to the bottom section, (3) molding the bottom section by two-layer injection molding in which the resin of the lower layer thereof is more rigid than the resin of the upper layer, (4) molding the bottom section by sandwich molding in which the resin of the skin layer is more rigid than the resin of the core layer, or (5) molding the bottom section from a resin less rigid than the resin of the side section.
According to the present invention, the resin rocker-panel molding, which is attached to the lower part of a vehicle side body, comprises at least a bottom section and a side section having prescribed rigidity, the side section being almost perpendicular to the bottom section. The bottom section has an energy conversion means which forcibly converts vibration energy generated on collision of foreign matter into heat energy. When foreign matter strikes against the bottom section to generate vibrations, the vibration energy is forcibly converted into heat energy and thus reduced sufficiently. That is, the noise (impact sound) generated by the vibration energy is abated, and the noise transmitted from the rocker-panel molding through the car body to the space in the car is lessened. Since the rocker-panel molding, especially the side section thereof, possesses prescribed rigidity, it is hardly damaged, maintaining its function, even if a stress is imposed thereon, for example, when a driver or a passenger touches on getting in or out of the car.
In the first embodiment, the function as an energy conversion means is achieved by making the bottom section thinner than the side section. Therefore, when foreign matter strikes against the bottom section, the bottom section is easily deformed by the vibration energy generated by the impact, resulting in generation of heat energy. That is, the vibration energy is forcibly converted into heat energy.
In the second embodiment, the energy conversion means is a member bonded to the bottom section. While the member is joined to the bottom section through planar contact macroscopically, the joint surface has microscopically countless points of contact. Therefore, when foreign matter strikes against the bottom section, the member bonded to the bottom section through point contact develops microscopic slide by the vibration energy. As a result of the microscopic slide between the bottom section and the bonded member, the vibration energy is forcibly converted to heat energy. Further, there is formed a microscopic layer of air between the bottom section and the bonded member. This air layer functions as a noise absorber.
According to the third embodiment, the energy conversion means is a two-layer molded bottom section comprising lower and upper layers, in which the lower layer is made of a more rigid resin material than the upper layer. When foreign matter strikes against the bottom section, the generated vibration energy, while being transmitted from the lower layer to the upper layer, is forcibly converted into heat energy mostly in the upper layer made of a relatively non-rigid resin material and is thereby reduced sufficiently. Further, since the lower layer is more rigid than the upper layer, foreign matter striking thereon is prevented from being buried into the lower layer.
According to the fourth embodiment, the energy conversion means is a sandwich-molded bottom-section comprising a skin layer and a core layer, in which the skin layer is made of a more rigid resin material than the core layer. When foreign matter strikes against the bottom section, the generated vibration energy, while being transmitted from the skin layer to the core layer, is forcibly converted into heat energy mostly in the core layer made of a relatively non-rigid resin material and is thereby reduced sufficiently. Further, since the skin layer is more rigid than the core layer, foreign matter striking thereon is prevented from being buried into the lower skin layer.
According to the fifth embodiment, the function as an energy conversion means is achieved by making the bottom section using a resin that is less rigid than the resin of the side section which should have prescribed rigidity. Therefore, when foreign matter strikes against the bottom section, the bottom section is easily deformed by the generated vibration energy, resulting in generation of heat energy. That is, the vibration energy is forcibly converted into heat energy.